This application claims priority with respect to British Application No. 0106331.2, filed Mar. 15, 2001.
This invention relates to vehicle axles, and is particularly concerned with fixed lorries and trailer vehicles provided with a supplementary suspension and axle system that is self-steering.
It is already known to provide a vehicle with a self-steering axle system, where the wheels are located on a pivotally mounted axle, such that the wheels can freely turn to follow the steered wheels of the vehicle as the vehicle, for example, progresses round a corner or bend in a road. Such self-steering axle assemblies provide notable advantages over fixed axle systems, by avoiding excessive tyre wear or damage to tyres caused by a transverse shear load applied to a tyre as it slides or bounces whilst progressing round a corner or bend.
Existing constructions of self-steering axles are quite adequate for all occasions (save perhaps for high vehicle speeds) whilst the vehicle is travelling in a forward direction, but suffer serious operational problems when the vehicle is required to be reversed, so much so that self-steering axles are frequently provided with a means of locking the axle against pivoting, when a reverse movement is needed, but with attendant wear on the tyres of the wheels on the self-steering axle.
The cause of this stems from the setting of pivot for the axle at an appreciable castor angle, to provide a line of action that projects from the axis of pivot through the road wheel centre and to a point on the tyre surface in advance of the tyre road contact. By providing the axle with an appreciable amount of castor angle, there is the substantial guarantee that the road wheels of the self-steering axle will hold in a straight line when the vehicle is travelling in a forward direction and the self-steering axle will follow the steered axle when the vehicle is travelling in a forward direction around a corner or bend.
The object of the present invention is to provide a means of eliminating those operational problems attendant to known supplementary suspension and axle systems when the vehicle is being reversed.
According to the present invention, a vehicle axle comprises a stub axle assembly mounted by a first pivot on a support, and the support being mounted on a second pivot on an axle beam, said support being provided with a means of causing it to pivot between two operational positions, and to hold the support in a required operational position, and said first and second pivots being disposed at a convergent angle whereby to set the first pivot at a first castor angle in one operational position, and at an equal and opposite second castor angle in the other operational position.
The particular means of causing the support to pivot and to hold it in its required operational position is not critical, but can conveniently be chosen to suit the power supply on the vehicle. Thus with vehicles with a compressed air system for such as the brakes, two air springs may be provided, one to either side of the support. With one air spring disconnected, the other can be activated to cause the support to pivot, and lock it in its required position. With vehicles having hydraulic systems, hydraulic piston and cylinders can be provided, single or double acting, to cause the support to pivot and lock it in its required position.